To the best of my knowledge it is not possible to conditionally select the dataset that you'd like to use for a single
Another solution (to avoid the expression based text boxes) would be to duplicate the summary line and then conditionally show/hide the tablix (so that only one shows, dependent on the parameter that is selected).
The expression for the visibility of the tablix could be something along the lines of
Conditional Statement for Tablix1 (which you want to show if Parameter1 is selected):
=IIF(IsNothing(@Parameter1), false, true)
Conditional Statement for Tablix2 (which you want to show if Parameter2 is selected):
=IIF(IsNothing(@Parameter2), false, true)